1. Field of the Invention
The present invention relates to a chain block and, more particularly, to a hand operated chain block wherein a load sheave is rotated to wind up and down a load chain passing over the load sheave by the operation of a hand chain passing over a hand wheel.
2. Description of the Prior Art
In general, hand operated chain blocks of this type comprise a load sheave supported between two opposed side plates via bearings; a hand wheel disposed at the outside of one side plate to drive the load sheave through a drive shaft and a reduction gear mechanism; and a transmission mechanism having a mechanical brake which is interposed between the hand wheel and the load sheave to apply a given brake and are so structured that a load suspended from a hook of the load chain passing over the load sheave can be hoisted up and lowered down by the operation of a hand chain passing over the hand wheel.
Of these known chain blocks, some are provided with an overload protection mechanism that permits the hand wheel to freely rotate in the hoisting direction when a load in excess of a rated load of the chain block is applied to the load sheave, for preventing the hoist of the load more than the rated load.
The overload protection mechanism comprises, for example, a hub member threadedly mounted on the drive shaft and supporting thereon the hand wheel in a freely rotatable manner; a load setting mechanism, arranged at an axially outside end portion of the hub member, for setting a rated load; and two lining plates interposed in a freely rotatable manner between the hub member and the hand wheel and between the hand wheel and the load setting mechanism, respectively. The overload protection mechanism is so structured that when the hand wheel is rotated in the hoisting direction in the state in which a load more than a rated load as is preset by the load setting mechanism pressing the lining plates is being applied, slippage of the hand wheel can be caused between the two lining plates placed at both sides of the hand wheel to permit free rotation of the hand wheel with respect to the hub member.
On the other hand, for example when the overload protection mechanism is put into action in the hoist of the load by the application of a load more than the rated load, the hand wheel is held slipped between the two lining plates placed at both sides thereof, and as such cannot permit the lowering of the load. For permitting the lowering of the load in such a situation, the hub member is provided, in an outer periphery thereof, with a recessed portion in which a pin and a biasing spring for biasing the pin radially outwardly are housed and also a boss of the hand wheel is provided, in an inner periphery thereof, with a receiving portion that can permit the pin to be retained therein only when the hand wheel is rotated in the lowering direction, whereby an one-way mechanism that permits free rotation of the hand wheel with respect to the hub member only in the hoisting direction and restricts the free rotation of the hand wheel in the lowering direction is formed so that the lowering of the load can be permitted even when a load more than the rated load is applied.
With this arrangement, when the hand wheel is rotated in the lowering direction in the state in which it is being slipped between the two lining plates placed at the both sides of the hand wheel by the action of the overload protection mechanism, the pin is smoothly received in the receiving portion but is sometimes caught in the receiving portion even when the hand wheel is rotated in the hoisting direction, depending on the form of the receiving portion. As a result of this, despite the load being more than the rated load, the load is sometimes hoisted in an unstable state, or the pin and the receiving portion are sometimes damaged. Also, there is presented a disadvantage that the hand wheel, when rotated, may rattle to cause damage to the hub member and the hand wheel. In the hand operated chain block of this type, in particular, even a little pulling of the hand chain may cause the hand wheel to rotate a couple of turns, and as such will give increased opportunities of the pin and the receiving portion to confront each other, thus significantly expanding the influence of the disadvantages above. Further, when it comes to a small-sized chain block having the hand wheel of a small diameter, even the little pulling of the hand chain will give further increased opportunities of the pin and the receiving portion to confront each other, thus further significantly expanding the influence.